There is an ever-increasing demand in recent years for water of high-purity to be used in the production processes or washing steps which are effected in the industries of electronics, chemicals, manufacture of precise-shaped parts and industry of foods. Moreover, such high-purity water is always required to be germ-free pure water. In the pure water-producing and feeding systems or plants employed in these industrial fields, pure water as produced by such systems is used for various purposes. In the industry of chemicals, for example, there is a tendency that the systems for production of pure water to be used in the dissolution of raw materials, washing, formulation of preparations and dilution would be bigger and more complex. This tendency is enhanced further when the existing systems are modified or extended.
Many tanks, valves, pumps, filters and lengthy pipes are used and arranged in the pure water-producing and feeding systems causing the systems to become larger and more complex as described above, so that an environment is provided where viable cells of, microorganisms can more readily grow and develop. Further, a pure water feed line of the pure water-producing system is provided with many branch pipings. Accordingly, the pure water feed line tends to have spots or places where pure water stagnates and does not flow smoothly, in other words, water-stagnant pockets, so that the viable cells are likely to grow therein. When the system is in operation, pure water does not flow but rather remains stagnant throughout the system, further promoting the growth of viable cells. To minimize this problem, some plants have arranged a recirculation line for returning pure water to a pure water-containing tank which is incorporated into the pure water-producing system so that pure water is steadily circulated through the system to eliminate any water-stagnant spots, thereby preventing the growth of the viable cells. In another instances, each point of use i.e. wherever it is used of pure water is arranged to be equipped with a refiltration unit.
In addition, the pure water feed line must be subjected to periodical sterilization to kill the viable cells. A conventional sterilization procedure is to treat pure water for 1 hour with an aqueous solution of 1-3 wt. % hydrogen peroxide or with an aqueous solution of 1-5 ppm of sodium hypochlorite. These conventional methods are however accompanied by the problems that a great volume of waste water effluent which requires further treatment thereof if discharged after the sterilization process and use of a strong oxidizing agent such as hydrogen peroxide or sodium hypochlorite may oxidize and decompose ion-exchange resins, membrane filters and the like as arranged in the pure water-producing system. The conventional methods are also accompanied by the drawback that the system must be washed with a great deal of pure water to discharge the microcidal agent after the completion of the sterilization process.
Other sterilization methods include steam sterilization and hot-water sterilization. These methods are not considered to be very economical when their energy efficiency is taken into consideration.
For the reasons mentioned above, a great deal of energy, cost, labor and operation time are still indispensable for the continual or constant feeding of germ-free pure water from an enlarged and complicated pure water-producing and feeding system which continues to become greater and more complex.
An object of the present invention is therefore to solve many of the above-mentioned problems which the conventional methods encounter upon sterilization of the system for producing and feeding a water of high-purity, and to provide such a method which is easy to practice and can continually feed the germ-free pure water even in an enlarged and complicated system.